In general, regulated emissions for internal combustion engines include carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and particulates. Such regulations have become more stringent over recent years. For example, the regulated emissions of NOx and particulates from diesel-powered engines are low enough that, in many cases, the emissions levels cannot be met with improved combustion technologies alone. To that end, exhaust after-treatment systems are increasingly utilized to reduce the levels of harmful exhaust emissions present in exhaust gas.
Conventional exhaust gas after-treatment systems include any of several different components to reduce the levels of regulated pollutants present in exhaust gas. For example, SCR catalysts are structured to convert NOx (NO and NO2 in some fraction) into nitrogen gas (N2) and water vapor (H2O). A reductant (typically ammonia (NH3) in some form) is added to the exhaust gas upstream of the catalyst. The NOx and NH3 pass over the catalyst and a catalytic reaction takes place in which NOx and NH3 are converted into N2 and H2O.
In many conventional SCR systems, NH3 is used as a reductant. Typically, pure NH3 is not directly used due to safety concerns, expense, weight, lack of infrastructure, and other factors. Instead, many conventional systems utilize diesel exhaust fluid (“DEF”), which typically is a urea-water solution. To convert the DEF into NH3, the DEF is injected into a decomposition tube through which an exhaust stream flows. The injected DEF spray is heated by the exhaust gas stream to vaporize the urea-water solution and trigger the decomposition of urea into NH3. The exhaust gas mixture, including the NH3 decomposed from the urea, further mixes while flowing through the decomposition tube and passes over the SCR catalyst, where the NOx and NH3 are converted primarily to N2 and H2O.